According to the U.S. Department of Energy, modern, wind-driven electricity generators were born in the late 1970's. See “20% Wind Energy by 2030,” U.S. Department of Energy, July 2008. Until the early 1970s, wind energy filled a small niche market supplying mechanical power for grinding grain and pumping water, as well as electricity for rural battery charging. With the exception of battery chargers and rare experiments with larger electricity-producing machines, the windmills of 1850 and even 1950 differed very little from the primitive devices from which they were derived. As of July 2008, wind energy provided approximately 1% of total U.S. electricity generation.
As illustrated in FIG. 1, most modern wind turbines typically have 3-bladed rotors 10 with diameters of 10-80 meters mounted atop 60-80 meter towers 12. The average turbine installed in the United States in 2006 can produce approximately 1.6 megawatts of electrical power. Turbine power output is controlled by rotating the blades 10 around their long axis to change the angle of attack (pitch) with respect to the relative wind as the blades spin around the rotor hub 11. The turbine is pointed into the wind by rotating the nacelle 13 around the tower (yaw). Turbines are typically installed in arrays (farms) of 30-150 machines. A pitch controller (for blade pitch) regulates the power output and rotor speed to prevent overloading the structural components. Generally, a turbine will start producing power in winds of about 5.36 meters/second (12 miles per hour) and reach maximum power output at about 12.52-13.41 meters/second (28-30 miles per hour). The turbine will pitch or feather the blades to stop power production and rotation at about 22.35 meters/second (50 miles per hour).
In the 1980s, an approach of using low-cost parts from other industries produced machinery that usually worked, but was heavy, high-maintenance, and grid-unfriendly. Small-diameter machines were deployed in the California wind corridors, mostly in densely packed arrays that were not aesthetically pleasing in such a rural setting. These densely packed arrays also often blocked the wind from neighboring turbines, producing a great deal of turbulence for the downwind machines. Little was known about structural loads caused by turbulence, which led to the frequent and early failure of critical parts. Reliability and availability suffered as a result.